W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 systems (International Mobile Telecommunication system), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4, Release 5 and Release 6.
A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
However, knowing that user and operator requirements and expectations will continue to evolve, the 3GPP has begun considering the next major step or evolution of the 3G standard to ensure the long-term competitiveness of 3G.
The 3GPP recently launched a study item “Evolved UTRA and UTRAN” better known as “Long Term Evolution (LTE)”. The study will investigate means of achieving major leaps in performance in order to improve service provisioning, and to reduce user and operator costs. It is generally assumed that Internet Protocols (IP) will be used in mobility control, and that all future services will be IP-based. Therefore, the focus of the evolution is on enhancements to the packet-switched (PS) domain of legacy UMTS systems.
The main objectives of the evolution are to further improve service provisioning, and reduce user and operator costs, as already mentioned. More specifically, some key performance, capability and deployment requirements for the long-term evolution (LTE) are inter alia:                significantly higher data rates compared to HSDPA and HSUPA (envisioned are target peak data rates of more than 100 Mbps over the downlink and 50 Mbps over the uplink),        Mean user throughput improved by factors 2 and 3 for respectively uplink (UL) and downlink (DL),        high data rates with wide-area coverage,        cell-edge user throughput improved by a factor 2 for uplink and downlink,        uplink and downlink spectrum efficiency respectively improved by factors 2 and 3,        significantly reduced latency in the user plane in the interest of improving the performance of higher layer protocols (for example, TCP) as well as reducing the delay associated with control plane procedures (for instance, session setup), and        stand-alone system operation in spectrum allocations of different sizes ranging from 1.25 MHz to 20 MHz.        
One other deployment-related requirement for the long-term evolution study is to allow for a smooth migration to these technologies.
The ability to provide high bit rates is a key measure for LTE. Multiple parallel data stream transmission to a single terminal, using multiple-input-multiple-output (MIMO) techniques, is one important component to reach this. Larger transmission bandwidth and at the same time flexible spectrum allocation are other pieces to consider when deciding what radio access technique to use. The choice of adaptive multi-layer OFDM (Orthogonal Frequency Division Multiplexing), Adaptive Multi Layer (AML)-OFDM, in downlink will not only facilitate to operate at different bandwidths in general but also large bandwidths for high data rates in particular. Varying spectrum allocations, ranging from 1.25 MHz to 20 MHz, are supported by allocating corresponding numbers of AML-OFDM subcarriers. Operation in both paired and unpaired spectrum is possible as both time-division and frequency-division duplex is supported by AML-OFDM.
OFDM with Frequency-Domain Adaptation
The AML-OFDM-based downlink has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 1 ms in order to minimize the radio-interface latency. In order to handle different delay spreads and corresponding cell sizes with a modest overhead, the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms very large cells, up to and exceeding 120 km cell radiuses, with large amounts of time dispersion can be handled. In this case, the length is extended by reducing the number of OFDM symbols in a sub-frame.
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to split the frequency band into a number of narrowband channels. Therefore, OFDM allows transmitting data on relatively flat parallel channels (subcarriers) even if the channel of the whole frequency band is frequency selective due to a multipath environment. Since the subcarriers experience different channel states, the capacities of the subcarriers may vary and permit a transmission on each subcarrier with a distinct data-rate. Hence, subcarrier wise (frequency domain) Link Adaptation (LA) by means of Adaptive Modulation and Coding (AMC) increases the radio efficiency by transmitting different data-rates over the subcarriers.
OFDMA allows multiple users to transmit simultaneously on the different subcarriers per OFDM symbol. Since the probability that all users experience a deep fade in a particular subcarrier is very low, it can be assured that subcarriers are assigned to the users who see good channel gains on the corresponding sub-carriers. When allocating resources in the downlink to different users in a cell, the scheduler takes information on the channel status experienced by the users for the subcarriers into account. The control information signaled by the users, i.e. CQI, allows the scheduler to exploit the multi-user diversity, thereby increasing the spectral efficiency.
Two different resource allocation methods can be distinguished when considering a radio access scheme that distributes an available frequency spectrum among different users as in OFDMA. The first allocation mode or “localized mode” tries to benefit fully from frequency scheduling gain by allocating the subcarriers on which a specific UE experiences the best radio channel conditions. Since this scheduling mode requires associated signaling (resource allocation signaling, CQI in uplink), this mode would be best suited for non-real time, high data rate oriented services. In the localized resource allocation mode a user is allocated continuous blocks of subcarriers.
The second resource allocation mode or “distributed mode” relies on the frequency diversity effect to achieve transmission robustness by allocating resources that are scattered over time and frequency grid. The fundamental difference with localized mode is that the resource allocation algorithm does not try to allocate the physical resources based on some knowledge on the reception quality at the receiver but select more or less randomly the resource it allocates to a particular UE. This distributed resource allocation method seems to be best suited for real-time services as less associated signaling (no fast COI, no fast allocation signaling) relative to “localized mode” is required.
The two different resource allocation methods are shown in FIG. 1 for an OFDMA based radio access scheme. As can be seen from the left-hand part of the figure, which depicts the localized transmission mode, the localized mode is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths (time/frequency bins) of a localized signal. On the other hand, as can be seen from the right-hand part of the figure, distributed mode is characterized by the transmitted signal having a non-continuous spectrum that is distributed over more or less the entire system bandwidth (time/frequency bins).
Evolved-MBMS
In the following there will be briefly described an exemplary multicast service to which the previous embodiments of the invention may be applied. As already mentioned such a multicast service can be the Multicast Broadcast Multimedia Service (MBMS)
There will be support for MBMS right from the first version of the LTE specifications. However, specifications for E-MBMS are in early stages. Two important scenarios have been identified for E-MBMS. One scenario is the single-cell broadcast, and the second one is the MBMS Single Frequency Network (MBSFN).
MBSFN is a new feature that is being introduced in the LTE specification. MBSFN is envisaged for delivering services such as Mobile TV using the LTE infrastructure, and is expected to be a competitor to DVB-H-based TV broadcast. In MBSFN, the transmission happens from a time-synchronized set of eNBs using the same resource block. This enables over-the-air combining, thus improving the Signal-to-Interference plus Noise-Ratio (SINR) significantly, compared to non-SFN operation. The Cyclic Prefix (CP) used for MBSFN is slightly longer, and this enables the UE to combine transmissions from different eNBs, thus somewhat negating some of the advantages of SFN operation. There will be six symbols in a slot of 0.5 ms for MBSFN operation versus seven symbols in a slot of 0.5 ms for non-SFN operation.
The overall user-plane architecture for MBSFN operation is shown in FIG. 8. 3GPP has defined a SYNC protocol between the E-MBMS gateway and the eNBs to ensure that the same content is sent over-the-air from all the eNBs. As shown in FIG. 8, the eBM-SC is the source of the MBMS traffic, and the E-MBMS gateway is responsible for distributing the traffic to the different eNBs of the MBSFN area. IP multicast may be used for distributing the traffic from the E-MBMS gateway to the different eNBs. 3GPP has defined a control plane entity, known as the MBMS Coordination Entity (MCE) that ensures that the same resource block is allocated for a given service across all the eNBs of a given MBSFN area. It is the task of the MCE to ensure that the RLC/MAC layers at the eNBs are appropriately configured for MBSFN operation. 3GPP has currently assumed that header compression for MBMS services will be performed by the E-MBMS gateway. Both single-cell MBMS and MBSFN will typically use point-to-multipoint mode of transmission.
However, HARQ operation will probably only be supported for single-cell MBMS transmissions.
For E-MBMS, the following definitions are introduced. FIG. 9 depicts the new MBMS definitions.                MBSFN Synchronization Area: An area of the network where all NodeBs/eNodeBs can be synchronized and perform MBSFN transmissions. MBSFN Synchronization Areas are capable of supporting one or more MBSFN Areas. On a given frequency layer, a NodeB/eNodeB can only belong to one MBSFN Synchronization Area. MBSFN Synchronization Areas are independent from the definition of MBMS Service Areas.        MBSFN Transmission or a transmission in MBSFN mode: a simulcast transmission technique realised by transmission of identical waveforms at the same time from multiple cells. An MBSFN Transmission from multiple cells within the MBSFN Area is seen as a single transmission by a UE.        MBSFN Area: an MBSFN Area consists of a group of cells within an MBSFN Synchronization Area of a network, which are co-ordinated to achieve an MBSFN Transmission. A cell within an MBSFN Synchronization Area may form part of multiple MBSFN Areas, each characterized by different transmitted content and participating set of cells.        MBSFN Area Transmitting and Advertising Cell: A cell within an MBSFN Area which is contributing to the MBSFN Transmission and advertises within the cell the availability of the MBSFN Transmission.        MBSFN Area Transmitting-Only Cell: A cell within an MBSFN Area which is contributing to the MBSFN Transmission but does not advertise within the cell the availability of the MBSFN Transmission. The need for this type of cell is FFS.        MBSFN Area Reserved Cell: A cell within an MBSFN Area, which does not contribute to the MBSFN Transmission. The cell may be allowed to transmit for other services but at restricted power on the resource allocated for the MBSFN transmission e.g. PTP for users at the centre of the cell.LTE Architecture        
The overall architecture is shown in FIG. 2 and a more detailed representation of the E-UTRAN architecture is given in FIG. 3. The E-UTRAN consists of evolved Node Bs (eNB), providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the mobile node (referred to in the following as UE or MN).
The eNB hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. Further, it performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL-QoS (Quality of Service), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME, and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs.
The S-GW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and Packet Data Network Gateway). For idle state UEs, the S-GW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the S-GW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the Home Subscriber Server, HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
L1/2 Control Signaling
In order to Inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ), L1/L2 control signaling needs to be transmitted on the downlink along with the data. The control signaling needs to be multiplexed with the downlink data in a sub frame (assuming that the user allocation can change from sub frame to sub frame). Here, it should be noted, that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling then needs only to be transmitted once per TTI. However, in some cases it may make sense to repeat the L1/2 control signaling within a TTI in order to increase reliability.
Generally, the information sent on the L1/L2 control signaling may be separated into the following two categories:                Shared Control Information (SCI) carrying Cat 1 information. The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:                    User identity, indicating the user which is allocated.            RB allocation Information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.            Optional: Duration of assignment, if an assignment over multiple sub frames (or TTIs) is possible.                        Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).        Dedicated Control Information (DCI) carrying Cat 2/3 information. The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1. The DCI typically contains information on:                    Cat 2: Modulation scheme, transport block (payload) size (or coding rate), MIMO related information, etc.            Cat 3: HARQ related information, e.g. hybrid ARQ process number, redundancy version, re-transmission sequence number.Hybrid ARQ Schemes                        
A common technique for error detection and correction in packet transmission systems over unreliable channels is called Hybrid Automatic Repeat reQuest (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which uses acknowledgments and timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the receiver to the transmitter to indicate that it has correctly received a data packet. If the sender does not receive an acknowledgment before the timeout, being a reasonable time interval for receiving an acknowledgment, it usually re-transmits the frame until it receives an acknowledgment or exceeds a predefined number of re-transmissions.
Forward error correction is employed to control errors in data transmissions, wherein the sender adds redundant data to its messages. This enables the receiver to detect whether an error has occurred, and further allows to correct some errors without requesting additional data from the sender. Consequently, since within a certain limit FEC allows to correct some of the errors, re-transmission of data packets can often be avoided. However, due to the additional data that is appended to each data packet, this comes at the cost of higher bandwidth requirements.
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by a CRC (Cyclic Redundancy Check)), the receiver requests a re-transmission of the packet.
Depending on the information (generally code bits/symbols) of which the transmission is composed of, and depending on how the receiver processes the information, the following hybrid ARQ schemes are defined:    Type I: The error detection information (such as CRC) is added to the data packet, which is then encoded with a forward error correction code (such as Reed-Solomon code or Turbo code). In the receiver the FEC code is decoded and the quality of the packet is determined. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can decode the actual data packet correctly. If the channel quality is bad and not all transmission errors can be corrected, then the received coded data packet is discarded and a re-transmission for said data packet is requested by the receiver. In this type of HARQ the re-transmission uses the same FEC code as during the initial transmission. Further, the re-transmission data packets contain identical information (code bits/symbols) to the initial transmission. Resulting from the above, the received transmissions are all decoded separately in the receiver.    Type II: According to the second type of HARQ, if the receiver fails to decode a packet correctly, the receiver stores the information of the (erroneously received) encoded packet as soft information (soft bits/symbols) and a re-transmission is requested from the sender. This implies that a soft-buffer is required at the receiver. Re-transmissions can be composed out of identical, partly-identical or non-identical information (code bits/symbols) compared to the initially transmitted data packet. When receiving a re-transmission the receiver combines the stored information from the soft buffer and the currently received information and tries to decode the packet based on the combined information. The combining of transmissions refers to so called soft-combining, where multiple received code bits/symbols are likelihood combined and solely received code bits/symbols are code combined. Common methods for soft-combining are Maximum Ratio Combining (MRC) of received modulation symbols and log-likelihood ratio (LLR) combining (LLR combining only works for code bits). Type II HARQ schemes are more sophisticated than Type I HARQ schemes, since the probability for a correct reception of a data packet increases with each received re-transmission. This increase comes at the cost of a HARQ soft-buffer at the receiver.            The Type II HARQ scheme can be used to perform dynamic link adaptation by controlling the amount of information to be re-transmitted. E.g. if the receiver detects that decoding has been almost successful, it can request only a small piece of information for the next re-transmission (smaller number of code bits/symbols than in the previous transmission) to be transmitted. In this case it might happen that it is even theoretically not possible to correctly decode the re-transmission packet by itself, wherein this is referred to as non-self-decodable re-transmissions.            Type III: This is a subset of the Type II HARQ with the restriction that each transmission, be it an initial or a re-transmission, must be self decodable.
The HARQ mechanism has been defined for unicast data transmissions, wherein there are two levels of re-transmissions for providing reliability, namely, the Hybrid Automatic Repeat reQuest (HARQ) at the MAC layer and the outer ARQ at the RLC layer.
However, recently the support of HARQ with soft-combining has been agreed in LTE for multicast transmission, such as MBMS (Multicast Broadcast Multimedia Services). UMTS doesn't support such functionality for multicast transmissions. Therefore, there are some technical challenges introduced, which need to be addressed. One of the problems is the soft-buffer management in case of simultaneously supporting HARQ operation with soft-combining for unicast and multicast transmissions.
Some simple solution would be to add some extra soft-buffer memory for the MBMS HARQ operation in the UE, in addition to the soft-buffer memory used for normal unicast HARQ protocol operation. This approach would not require any changes in the UE respectively eNB behaviour with regard to the HARQ protocol. However, since soft-buffer memory in a mobile terminal is very expensive, this solution might not be very sensible.